The present invention relates to a method and apparatus for calcination, and more especially to a method and apparatus for calcining calcium sulphate dihydrate or gypsum, and similarly heat-reactive minerals.
A method and apparatus for calcining gypsum is described in GB No. 2 027 859, in which a single bed of gypsum is heated by indirect heat exchange with burning fuel in a fluidized bed combustion unit and the gypsum is simultaneously fluidized and directly heated by the resulting combustion gases. We have found that hemihydrate obtained by this process requires the use of substantial quantities of a retarding agent to adjust the setting time to a convenient value for building plaster formulations.
In addition, we have found that some of the gypsum fed to the treatment bed of the single stage calciner described in GB No. 2 027 859 tends to pass from the inlet directly to the outlet with the result that a significant proportion of the gypsum leaves the calciner untreated and persists as dihydrate in the product. Paradoxically, some material tends to be retained in the bed for prolonged periods and this leads to the formation of soluble anhydrous calcium sulphate which again emerges in the product. Both of these by products reduce the quality of the stucco produced.